Bianchi Oltre XR4 fork offset (WARNING - TOTALLY OTT NERDY RANT)
neeb
Posts: 4,473
It's been seriously p*ssing me off that some bike manufacturers flat-out refuse to provide full geometry information for their framesets. If I'm contemplating spending £3000+ on a bike frame it's not unreasonable to want to know the trail, for example. Some manufacturers such as Specialised are quite good at providing this info (their charts include both fork offset and trail, as well as all of the other dimensions and stack and reach too). Others are not.. I have literally been trying for months to find out fork offset and/or trail for the various frame sizes of Bianchi Oltre XR4. It's become an obsession.. ;-) Bianchi tell you to ask a dealer. Dealers don't know and tell you to ask Bianchi. Finally, after a lot of hassle and blatant evasion from Bianchi I have had this response from them via social media:
"Bianchi office answered us. The informations you are looking for, are not issued by Bianchi."
Yes, that's right - I'm not allowed as a potential customer to know the trail of a three grand frameset I'm considering purchasing, widely acknowledged to be one of the most important aspects of geometry for the way a bike handles..
My strong suspicion was that this is because they save costs by specifying the same fork across the size range, resulting in wacky trail figures for some frame sizes. Not to be put off, I was determined to try to find out, and I think I have - but I'd be happy to have someone potentially more knowledgable or more geometrically gifted check my sums!
Here is Bianchi's geo chart for the XR4:
You can easily calculate trail from fork offset and head tube angle, but with one of these missing it's rather difficult.. I found a way, but because it's rather roundabout and numbers in these charts are rounded up to the nearest mm there are rounding errors in the results. However, I believe the figures as a whole are consistent enough to strongly suggest that Bianchi uses a 45mm rake fork across the entire range of sizes.
Assuming a size 550:
Dimension F in the chart gives the BB to front hub distance. Along with the BB drop of 68mm this gives the horizontal dimension (in green) of 584mm.
Subtracting reach (X) from that gives 194mm as the horizontal distance between the top of the head tube and the front hub, and then the vertical dimension of 473mm from stack (Y) minus BB drop.
Together with head tube angle (G1), this allows the calculation of the horizontal dimension of 149mm. The difference between this and 194mm is the fork offset - 45mm in this case.
Doing this for all frame sizes gives the following figures for offset:
470 46
500 46
530 45
550 45
570 44
590 46
610 43
I don't believe that Bianchi make four different forks differering by only 3mm between them and specify the same one on a size 470 and a size 590, therefore my strong suspicion is that the same size is used across the range (probably a 45mm), and that the 1mm variations from this (2mm in the case of size 610) are due to rounding errors. If that's the case, the figures for trail are as follows:
Frame Size | Assumed offset | Trail
470 | 45 | 73
500 | 45 | 66
530 | 45 | 63
550 | 45 | 60
570 | 45 | 57
590 | 45 | 57
610 | 45 | 57
This would mean that the medium and large frame sizes have fairly normal trails, but the smaller sizes, and the smallest two in particular, have fairly extreme trails on the longer side (leading to very unresponsive steering in many situations). 60mm for size 550 (my size) is also just a little bit more than I would ideally choose, which is the simple information I wanted to know in the first place...
Admittedly these figures aren't radically different from many other bike frames - small frames always seem to end up with large trails, although at least some manufacturers try to mitigate it by specifying 2 different rakes of fork across the range. It just annoys me that Bianchi aren't up front about this!
Yes, I have too much time on my hands - courtesy of a broken rib and not having been able to go riding for the past 12 days...
"Bianchi office answered us. The informations you are looking for, are not issued by Bianchi."
Yes, that's right - I'm not allowed as a potential customer to know the trail of a three grand frameset I'm considering purchasing, widely acknowledged to be one of the most important aspects of geometry for the way a bike handles..
My strong suspicion was that this is because they save costs by specifying the same fork across the size range, resulting in wacky trail figures for some frame sizes. Not to be put off, I was determined to try to find out, and I think I have - but I'd be happy to have someone potentially more knowledgable or more geometrically gifted check my sums!
Here is Bianchi's geo chart for the XR4:
You can easily calculate trail from fork offset and head tube angle, but with one of these missing it's rather difficult.. I found a way, but because it's rather roundabout and numbers in these charts are rounded up to the nearest mm there are rounding errors in the results. However, I believe the figures as a whole are consistent enough to strongly suggest that Bianchi uses a 45mm rake fork across the entire range of sizes.
Assuming a size 550:
Dimension F in the chart gives the BB to front hub distance. Along with the BB drop of 68mm this gives the horizontal dimension (in green) of 584mm.
Subtracting reach (X) from that gives 194mm as the horizontal distance between the top of the head tube and the front hub, and then the vertical dimension of 473mm from stack (Y) minus BB drop.
Together with head tube angle (G1), this allows the calculation of the horizontal dimension of 149mm. The difference between this and 194mm is the fork offset - 45mm in this case.
Doing this for all frame sizes gives the following figures for offset:
470 46
500 46
530 45
550 45
570 44
590 46
610 43
I don't believe that Bianchi make four different forks differering by only 3mm between them and specify the same one on a size 470 and a size 590, therefore my strong suspicion is that the same size is used across the range (probably a 45mm), and that the 1mm variations from this (2mm in the case of size 610) are due to rounding errors. If that's the case, the figures for trail are as follows:
Frame Size | Assumed offset | Trail
470 | 45 | 73
500 | 45 | 66
530 | 45 | 63
550 | 45 | 60
570 | 45 | 57
590 | 45 | 57
610 | 45 | 57
This would mean that the medium and large frame sizes have fairly normal trails, but the smaller sizes, and the smallest two in particular, have fairly extreme trails on the longer side (leading to very unresponsive steering in many situations). 60mm for size 550 (my size) is also just a little bit more than I would ideally choose, which is the simple information I wanted to know in the first place...
Admittedly these figures aren't radically different from many other bike frames - small frames always seem to end up with large trails, although at least some manufacturers try to mitigate it by specifying 2 different rakes of fork across the range. It just annoys me that Bianchi aren't up front about this!
Yes, I have too much time on my hands - courtesy of a broken rib and not having been able to go riding for the past 12 days...
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Comments
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You could probably sort out your rounding errors by building a simple model here
https://www.bikecad.ca/quickapplet
It won't change anything though and it might not make you feel any happier.
I'd recommend test riding one before making too firm an assumption of the ride feel based on the trail number.0 -
Barely read any of the above but when spending £3k on a frameset it's a heart over head purchase and you have to have a warm fussy glow about the manufacturer. With shoddy customer services before you've even bought it, why not move on to something else?0
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Wow that looks really useful, thanks!Dave334 wrote:You could probably sort out your rounding errors by building a simple model here
https://www.bikecad.ca/quickapplet0 -
I’m really fussy, it has to be a heart + head purchase..londoncommuter wrote:Barely read any of the above but when spending £3k on a frameset it's a heart over head purchase and you have to have a warm fussy glow about the manufacturer. With shoddy customer services before you've even bought it, why not move on to something else?
I’ve pretty much already moved on, the Oltre’s also got a dedicated seatpost that isn’t ideal for me - it’s reversible, but really what I need is an inline.
At the moment the Ridley Helium SLX is growing on me. Looks much nicer in the flesh than in photos, especially in the slightly smaller-medium sizes. Extremely nice ride by all accounts. Only thing is it’s probably as aero as a brick..0 -
U ok hun?0
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the smaller sizes, and the smallest two in particular, have fairly extreme trails on the longer side (leading to very unresponsive steering in many situations)... Admittedly these figures aren't radically different from many other bike frames - small frames always seem to end up with large trails
Perhaps Bianchi don't bother publishing these figures because in the real world they don't actually matter that much, and a longer trail for shorter sizes might even be desirable.
For one, as you say, this seems to be standard across many manufacturers.
Secondly, there are other factors that will affect the steering response. For example, small riders / frames generally use shorter stems than tall riders on bigger frames, and a shorter stem will have the effect of sharpening up the steering, whilst a longer one slows it down. So overall the steering response across the sizes is likely to me much the same."an original thinker… the intellectual heir of Galileo and Einstein… suspicious of orthodoxy - any orthodoxy… He relishes all forms of ontological argument": jane90.0 -
I had a bike once that had a relatively long trail, which for me at least meant that I had no confidence descending - it seemed that it just didn't turn very well at speed, or at least not in a way that felt safe.
I know what you mean about shorter stem length and longer trail to some extent compensating for each other, but they're not the same thing - a longer stem just acts as a longer lever giving you more fine control over a twitchy short trail, but (as I understand it) it doesn't affect inherent stability (i.e. minus the steering input).
I'd be interested to know what an expert frame buider would ideally specify for a shorter person in terms of trail and stem length. The slacker head tube angles on small frames are largely to prevent toe overlap. In terms of trail this can in theory be completely compensated for by specifying a fork with a large enough offset. Yet on off-the-peg frames you never seem to see XXS size frames with the 55-60mm of trail that is often touted as "ideal". I'd always assumed that this was because even if there are two fork rakes across the size range, the larger offset one is stil going to be on the smaller side for the smallest frame size. Maybe a shorter stem means that a slightly larger trail is ideal, but I doubt that the ideal is going to be in the mid 70s..
My own experience is that trail does make a big difference in the real world, but in any case the point is that if you are buying a top end frameset for multiple thousands of pounds it's just rridiculous not to be provided with complete geometry information. I don't believe for one moment that Bianchi don't know the offsets of their forks - they clearly have a reason for not releasing this information.0 -
If there really was such an immutable relationship between head tube angle and rake, it makes one wonder how all those old timers ever managed to get around the corners, given the huge amount of fork rake that was once fashionable.
"an original thinker… the intellectual heir of Galileo and Einstein… suspicious of orthodoxy - any orthodoxy… He relishes all forms of ontological argument": jane90.0 -
Yes, well it was fashionable in those days to have massive fork rakes and steep head tube angles, so actually very little trail - the bikes were very twitchy..BenderRodriguez wrote:If there really was such an immutable relationship between head tube angle and rake, it makes one wonder how all those old timers ever managed to get around the corners, given the huge amount of fork rake that was once fashionable.0 -
Have you tried getting in touch with Epic Cycles? They're a Bianchi stockist and they've been really helpful whenever I've contacted them.0
